Method for forming a spacer for semiconductor manufacture

ABSTRACT

This invention teaches methods and apparatus for forming self-aligned photosensitive material spacers about protruding structures in semiconductor devices. One embodiment of the invention is a method for forming a LDD structure, utilizing disposable photosensitive material spacers. A second embodiment of the invention comprises a method for forming a transistor, having salicided source/drain regions, utilizing photosensitive polyimide spacers for forming the salicided source/drain regions, without disposing of the spacers. A third embodiment of the invention comprises a method for creating an offset from a protruding structure on a semiconductor substrate, using disposable photosensitive material spacers.

This application is a divisional of U.S. Ser. No. 08/661,795, filed Jun.13. 1996.

This invention was made with government support under Contract No.MDA972-92-C-0054, awarded by Advanced Research Projects Agency (ARPA).The Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to methods for forming photosensitive materialspacers in the manufacture of semiconductor devices.

BACKGROUND OF THE INVENTION

Semiconductor processing often requires spacers for ion implantation.Spacers have been used in process steps, such as transistorlightly-doped drain (LDD) formation and source/drain implantation. LDDsare utilized to reduce hot electron effects in MOS devices. Thesestructures absorb some of the potential in the drain and reduce theresulting electric field. Reducing the electric field also reduces hotelectron-induced gate currents, increasing device stability.

In the past, nitride and oxide materials have been utilized for LDDfabrication spacers. Two source/drain implantations are done afterformation of a gate. Source/drain regions immediately adjacent to thegate are lightly-doped, and source/drain regions farther from the gateare heavily-doped. Spacers are formed alongside the gate after a lightsource/drain implantation. Then, a second ion implantation formsheavily-doped regions within the already implanted source/drain regions,farther from the gate. However, spacers can be formed prior to the lightsource/drain implantation. Then, the source/drain region isheavily-doped with an implantation adjacent to the spacers.Subsequently, the spacers are removed and a lightly-doped implant regionis formed adjacent to the gate.

Oxide spacers are often utilized in the formation of self-alignedsource/drain regions in metal-oxide-semiconductor (MOS) devices.Self-aligned source/drain silicide (salicide) films are utilized todecrease circuit resistance in devices. As devices shrink, circuitresistance increases. Furthermore, sheet resistivity ofshallow-junctions of source/drain regions also increases. Therefore,saliciding processes attempt to overcome this increased resistance.Spacers are formed alongside the gate after source/drain implantation.Then, a refractory metal silicide is formed alongside the spacers.Silicide can be formed in a variety of ways, such as by depositing alayer of refractory metal and annealing, or depositing a refractorymetal silicide. Subsequent contacts to the silicided source/drainregions have decreased resistance throughout the contact area

The common process flow to form a spacer is first to deposit a conformalfilm, like oxide or nitride, followed by a dry etch. Due to the dry etchprocess step, the silicon substrate and gate oxide integrity may bedegraded. As a result, damaged layers will etch at a faster rate,undesirably altering the thickness of the layers. Another limitation ofusing oxide or nitride for spacer material is that such layers are oftendeposited using a high temperature deposition step, which may causeundesirable dopant migration, reflow at undesired times, or otherunwanted effects in surrounding device areas. Another problem with usingoxide and nitride films for spacer material is that they may not alwaysbe removed after the implantation step. Ions implanted into such layersdiffuse during subsequent thermal process steps. Thus, if such layersare not of adequate thicknesses, it is hard to control the diffusion ofunwanted impurities into device regions masked by the spacers.

There is a need for a spacer material which does not subject surroundingdevice regions to implantation damage or damage caused by dry etching toform the spacer, as in the case of oxides and nitrides. There is a needfor a spacer, which is easy to define on a substrate without the needfor precise masking steps. There is a further need for a spacer materialthat does not require high temperature deposition and is easily removedafter its use.

SUMMARY OF THE INVENTION

Spacers are formed in semiconductor devices by controllably exposing anddeveloping a photosensitive material, such that spacers remain,self-aligned with structures formed on substrates. Further processingsteps, such as ion implantation, are then performed, with the unexposedphotosensitive material masking out the ions. The spacers can then beremoved using wet chemical etches. The formation of the spacer does notrequire high temperatures and, thus, does not damage the substrate. Inaddition, the spacers are self-aligned with the edges of the structures(topography).

In one embodiment of the invention, a method for forming a lightly-dopeddrain (LDD) structure on a semiconductor wafer comprises the steps of:defining active areas on the wafer; forming at least one gate on thesemiconductor wafer in a defined active area; depositing aphotosensitive material onto the wafer; controllably exposing thephotosensitive material; developing the photosensitive material to format least one spacer alongside the gate; implanting a first dose of ionsinto the active areas; removing the spacers; and implanting a seconddose of ions into the active areas.

In another embodiment of the invention, a method for forming a LDDstructure on a semiconductor wafer comprises the steps of: definingactive areas on the wafer; forming at least one gate on thesemiconductor wafer, in a defined active area; implanting a first doseof ions into the active areas, adjacent to the gate; depositing aphotosensitive material onto the wafer; controllably exposing thephotosensitive material; developing the photosensitive material to format least one spacer alongside the gate; implanting a second dose of ionsinto the active area; and removing the spacers.

In another embodiment of the invention, a method for forming atransistor, having salicided source/drain regions on a semiconductorwafer, comprises the steps of: defining active areas; forming at leastone gate on the semiconductor wafer in a defined active area; dopingsource/drain regions; depositing photosensitive polyimide onto thewafer; controllably exposing the polyimide; developing the polyimide toform self-aligned spacers alongside the gate; and forming silicide onthe source/drain regions. The spacers are not removed in this embodimentof the invention because polyimide is more stable at high temperaturesthan other photosensitive material, such as conventional photoresist.

In yet another embodiment of the invention, disposable spacers, formedfrom unexposed photosensitive material, are utilized to create offsetsfrom protruding structures on a substrate. Use of such disposablespacers permits the formation of complex topographies due to their easeof fabrication and removal.

Spacers, formed from unexposed photosensitive material, are much easierto use than prior art spacers, such as oxide and nitride spacers. Suchspacers do not require dry etching for their formation, as do oxide andnitride spacers. Thus, the problem of dry etch-induced lattice damage tosurrounding material, such as the gate oxide or silicon substrate, isnot present when using spacers described in the invention. Anotheradvantage of using such a spacer is that it does not require a hightemperature deposition step and it is easily removed. Due to diffractionand phase shift along the edges of structures during exposure of thephotosensitive material, the spacers are formed self-aligned with thestructures. No mask or precise alignment of a mask is required, but maybe used if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration representing a LDD structure,fabricated utilizing disposable photosensitive material spacers.

FIGS. 2a-2 d are cross-sectional illustrations representing processsteps utilized to form the disposable photosensitive material spacersfor the LDD structure.

FIG. 3 is a cross-sectional illustration representing salicidedsource/drain regions in a MOS transistor, fabricated utilizing spacers,formed from photosensitive material.

FIGS. 4a and 4 b are cross-sectional illustrations representing processsteps utilized to form the photosensitive material spacers for the MOStransistor structure shown in FIG. 3.

FIGS. 5a-5 c are cross-sectional illustrations representing the processsteps utilized to create an offset from a protruding structure, usingdisposable photosensitive material spacers.

DETAILED DESCRIPTION

This invention overcomes problems associated with spacers utilized inthe past, by utilizing photosensitive material to form such spacers,self-aligned with topographical structural edges in varioussemiconductor processing steps. By utilizing photosensitive material forspacers, a high temperature deposition step is not required, and dryetching is not required to define the spacers. Therefore, the substrateis not damaged during spacer formation. Problems associated withimplantation damage are not present when photosensitive material is usedas the spacer material. Furthermore, the spacers are easily removed byan oxygen plasma, followed by a wet chemical, such as sulfuric acid, ora hydrogen peroxide solution.

Disposable photosensitive material spacers are utilized in thefabrication of lightly-doped drain (LDD) structures. One such structureis shown in FIG. 1. A gate 112 is formed over a gate oxide layer 108.The source/drain 110 of an LDD structure is then formed using twosubsequent implants and disposable photosensitive material spacers. Thefirst implant is self-aligned with gate 112 and spacers (not shown)formed on both sides of the gate 112. The second implant is self-alignedwith only the gate 112, following removal of the spacers. The result isformation of a lightly-doped section 114 of the source/drain 110, at theedge near the gate 112 and a more heavily-doped section 116 next to thatregion. Subsequent drive-in steps cause the implanted dopants to spreadout by diffusing through the substrate 120. Thus, as FIG. 1, thelightly-doped 114 and heavily-doped regions 116 are not always confinedto the initial implant areas. Remaining elements of the LDD structureillustrated in FIG. 1 are well known in the art and are comprised ofoxide 118 surrounding the gate 112 and non-active areas of the device,and a semiconductor substrate 120.

FIGS. 2a-2 d illustrate the process steps for forming LDD structures. Toform removable spacers on edges of semiconductor structures, a siliconsubstrate is primed using hexamethyldisilazane (HMDS) vapor at 120degrees Celsius, prior to coating with a photosensitive material 226,such as photoresist. However, priming is not always necessary, and otheragents can be used at different temperatures. Although photoresistmaterial is described in this example, other types of photosensitivematerial responsive to exposure sources, such as x-rays, ions, andelectrons are used in further embodiments, to expose the material,instead of using ultraviolet photons. Such other photosensitivematerials and ways of exposing them are well known to one skilled in theart.

To form disposable spacers, photosensitive material 226 is applied to awafer 222, having a protruding structure 224, using conventional spinnertechniques, as shown in FIG. 2a. In the case of LDD structure formation,the protruding structure 222 is a transistor gate formed over a gateoxide layer 208. The dimensions of the structure may vary, but the gatehas a gate height 230, which is the controlling structural variable. Thephotosensitive material 226 is deposited to a photosensitive materialheight, which is equal to or greater than the gate height 230. In thepreferred embodiment, the photosensitive material height is at least 0.5microns thicker than the gate height 230. However, depending on thedevice and application, these dimensions may vary further, and thephotosensitive material height may be less than the gate height 230. Asdevices become increasingly smaller, such heights are expected todecrease. Varying the heights of the gate and photosensitive material donot depart from the scope of this invention.

The wafer 222 is spun under controlled velocity and acceleration,spreading photosensitive material 226 in a nearly uniform layer, asshown in FIG. 2b. For example, positive photoresist is used, comprisedof a diazonaphtoquinone-type sensitizer in a novolak base resin.Specifically, Sumitomo PF138A is used. However, any photosensitivematerial suitable for wafer processing can be used. Various parametersaffect the resulting photosensitive material layer properties, includingthe dimensions of the structure, the energy of ions subsequentlyimplanted, and the type of ions subsequently implanted. However, thephotosensitive material height 226 should be approximately 0.5 micronshigher than the gate height 230, as shown in FIG. 2a. Following thephotosensitive material coating, a soft bake is performed at 90 degreesCelsius, for approximately 60 seconds.

The photosensitive material 226 is next exposed using techniques wellknown in the art, as shown in FIG. 2c. Such techniques include usingcontact printers, steppers, and flood ultraviolet (UV) irradiation. Whenusing flood UV irradiation, source 228 exposes the photosensitivematerial layer 226, to pattern the photosensitive material 226 to beremoved. Due to diffraction along the edges of the gate 224 and thephase-shift effect due to the gate height 230, the photosensitivematerial 226 along the gate 224 edges receive a lower exposure than thephotosensitive material 226 covering the gate 224, or other areas of thewafer 222. The wavelength of the UV light used ranges from betweenapproximately 157 to 436 nanometers.

UV exposure time and development time are carefully monitored to formspacers 232 alongside the gate 224, as shown in FIG. 2d. Exposure to UVlight destroys the sensitizer in the photosensitive material, which is aphotoactive compound (PAC). Destroying the PAC causes optical bleaching.Values of bleachable absorption coefficient, the non-bleachableabsorption coefficient, and the bleach rate affect the exposure timeneeded. Destroying the PAC also increases the solubility rate of thenovolak resin in basic developers. Thus, development time is directlycorrelated with exposure time. Photosensitive material thickness plays arole in the exposure time needed for forming spacers. Light intensitydecreases with depth due to light absorption by the photoresist.Therefore, thicker photosensitive material layers require longerexposure times. Intensity at the photosensitive material surface alsoaffects exposure time needed. Adjustments may be made to these processparameters without departing from the scope of the invention. Apost-exposure bake at 115 degrees Celsius for approximately 60 secondsis then done. This post-exposure bake is not required for all types ofphotosensitive materials used.

The width 234 of the spacers 232 varies according to the exposure time,development time, and gate height 230, as shown in FIG. 2d. Adjustmentsare made to the process variables to achieve spacers of differentwidths, depending on ones preference and needs.

When using a stepper for exposing the photosensitive material, anASML/100 i-line stepper can be used. A wavelength of 365 nanometers isused with a numerical aperature (NA) of 0.54, with an energy of 110mJ/cm². The partial coherence or filling factor, sigma, is 0.62. Apost-exposure bake at 115 degrees Celsius for approximately 60 secondsis then performed. This post-exposure bake is not required for all typesof photosensitive materials used.

The photosensitive material is then developed using an MIF300 ShipleyDeveloper, based on tetramethyl ammonium hydroxide (TMAH) in a watersolution, for approximately 60 seconds. Other developing solutions andequipment can be used, as is well known to one skilled in the art Theresult is that spacers are formed self-aligned about the edges ofstructures without having to perform precise alignments and generationof a mask. However, a mask may be used where a spacer is not desiredadjacent all protruding structures.

Subsequent processing steps for forming LDD structures comprise thefollowing: implanting a first dose of ions to a first concentrationlevel, removing the spacers, and implanting a second dose of ions to asecond concentration level. In a typical NMOS device, the first dose ofions is typically arsenic at a dose of about 1-5×10¹⁵ arsenic atoms/cm².The second dose of ions is typically phosphorus at a dose of about1-2×10¹³ phosphorus atoms/cm². In a typical PMOS device, the first doseof ions is typically boron at a dose of about 1-5×10¹⁵ boron atomscm².The second dose of ions is typically boron at a dose of about 1-2×10¹³boron atoms/cm². Photosensitive material spacers are easily removedusing any of a number of conventional removal techniques. These includeorganic strippers, oxidizing-type strippers, or dry etching techniques.Commonly used organic strippers are comprised of: phenol-basedstrippers, low-phenol, and phenol-free organic strippers. Commonly usedoxidizing-type strippers are comprised of sulfuric acid and hydrogenperoxide solutions. Dry etching is usually performed using a dry oxygenplasma In this example, an oxygen plasma etch is followed by applicationof a wet sulfuric and hydrogen peroxide mixture at 120 degrees Celsius.

In another embodiment of the invention for forming LDD structures,spacers are formed in accordance with the method recited above, but notuntil after a light, first source/drain implantation is done. In atypical NMOS device, this first dose of ions is typically phosphorus ata dose of about 1-2×10¹³ phosphorus atoms/cm². Subsequent to forming theresist spacers, a heavy, second source/drain implantation forms aheavily-doped region farther away from the gate than the lightly-dopedregion formed with the first implantation. In a typical NMOS device,this second dose of ions is typically arsenic at a dose of about1-5×10¹⁵ arsenic atoms/cm². PMOS devices are formed in similar ways,except for changes in implanted species and doses. In a typical PMOSdevice, the first dose of ions is typically boron at a dose of about1-5×10¹⁵ boron atoms/cm². The second dose of ions is typically boron ata dose of about 1-2×10¹³ boron atoms/cm². The spacers are removed inaccordance with the method recited above. An insulating layer is thendeposited over the entire structure.

In another embodiment of the invention, photosensitive material spacers340 are utilized in forming salicided source/drain regions 342 of a MOStransistor 344, as shown in FIG. 3. Spacers 340 are formed on the sidesof gate 346, overlying a gate oxide layer 308 and semiconductorsubstrate 348. The spacers 340 control the distance of the salicidedregions 342 from the gate 346, within the source/drain regions 350.Spacers 340 are formed out of a photosensitive polyimide. A typicalphotosensitive polyimide is comprised of polyimic acid, polyimic ester,PAC, and a solvent, such as normal methylpyrrolidone (NMP). When formingsilicided source/drain regions 342 in MOS transistors, as shown in FIG.3, by depositing titanium and annealing to form titanium silicide 342,high temperature stability is required. Photosensitive polyimide spacers340 are more stable at high temperatures than conventional photoresist.Therefore, they do not need to be removed. Such polyimide spacers 340can be used in other applications, where it is necessary that thespacers 340 remain on a structure during subsequent high temperatureprocessing steps. Subsequent process steps include forming metalcontacts 354 over the contact areas, which extend to the salicidedsource/drain regions 342. Field oxide 356 isolates each transistor 344on a die.

The steps necessary for fabricating spacers formed from photosensitivematerial in the MOS transistor, shown in FIG. 3, are illustrated inFIGS. 4a and 4 b. A gate 446 on a gate oxide layer 408 and semiconductorsubstrate 448, field oxide 456, and doped source/drain regions 450 arecreated utilizing conventional techniques, as shown in FIG. 4a.Salicided source/drain regions 442 are then formed by creating spacers440 alongside the gate 446, as shown in FIG. 4b.

The steps needed to form the spacers 440, as shown in FIG. 4b, aresimilar to those utilized in forming the spacers for the LDD structure.Photosensitive material is applied to the transistor base structureusing the same methods as described previously with LDD structures. Aspreviously stated, the height 458 of the gate, as shown in FIG. 4a, isthe controlling structure variable for forming the spacers.

Subsequent processing steps in forming MOS transistor structures, asshown in FIG. 3, comprise the following: depositing an insulating layer352 over the device, etching the insulating layer 352 to form contactholes, depositing metal over the device, and etching the metal to definemetal contact lines 354.

In another embodiment of the invention, photosensitive material spacersare utilized to create offsets from certain structures, including thosewhich have complicated topographies. The structure can be any protrudingstructure, such as a gate in a transistor or a silicon substrate recess,prior to field oxidation. The invention is used to obtain an offset 590from a structure 592 as shown in FIGS. 5a-5 c. A substrate 594 isprovided, on which a protruding structure 592 is defined. The substrate594 and the structure 592 can be of the same type of material, but it isnot necessary for the practice of this invention.

Spacers 596 are created on the sides of the structure 592 by the methoddescribed previously for LDD structures, as shown in FIG. 5a. Portionsof the substrate 594 are removed by either a wet or dry etch, as wellknown to one skilled in the art and shown in FIG. 5b. The spacers 596are then removed, leaving an offset 590 from the structure 592, as shownin FIG. 5c. Disposable photosensitive material spacers 596 are capableof forming such complex topographies due to their ease of fabricationand disposal.

In the above-described embodiments, spacers, formed from photosensitivematerial, provide many advantages over prior art spacers, such as oxideand nitride spacers. Such spacers do not require dry etching for theirformation, as do oxide and nitride spacers. Thus, the problem of latticedamage to surrounding material, such as the gate oxide or siliconsubstrate, is not present when using the spacers of the invention.Another advantage of using a disposable spacer is that it does notrequire a high temperature deposition step and it is easily removed.However, when subsequent high temperature steps are required, polyimidespacers are a suitable material, which do not need to be removed.

It should be noted that in MOS technology, many times certain areas ofthe semiconductor die described as having a particular doping, couldquite easily be of a different doping, promoting a different type ofcharge carrier. In such instances, if one were to reverse the primarycarriers in all areas of the die and adjust for carrier mobility, theinvention would operate in an equivalent manner.

What is claimed is:
 1. A method for forming a LDD structure on asemiconductor wafer, utilizing disposable photosensitive materialspacers, comprising: forming a gate on the semiconductor wafer, over adefined active device area; forming a photosensitive material spaceralongside the gate, wherein forming the photosensitive material spacerincludes coating the gate with a layer of photosensitive materialsuitable for use as the spacer, flood exposing the photosensitivematerial for a controlled length of time, and developing thephotosensitive material, wherein both flood exposing the photosensitivematerial for a controlled length of time, and developing thephotosensitive material avoid the use of a photo mask; implanting afirst dose of ions into the active device areas; removing thephotosensitive material spacer; and implanting a second dose of ionsinto the active device areas.
 2. The method of claim 1, wherein thephotosensitive material spacers are removed by an oxygen plasma etch,followed by application of a wet sulfuric and hydrogen peroxide mixtureat approximately 120 degrees Celsius.
 3. The method of claim 1, whereincoating the gate with a layer of photosensitive material includesdepositing the photosensitive material onto the wafer such that theheight of the photosensitive material is at least approximately 0.5microns greater than the gate height.
 4. The method of claim 1, whereincoating the gate with a layer of photosensitive material includes bycoating tie gate with a polyimide material.
 5. The method of claim 1,wherein removing the spacers includes using wet chemicals.
 6. A methodfor forming a LDD structure on a semiconductor wafer, utilizingdisposable photosensitive material spacers, comprising: forming a gate,having edges and a gate height, on the semiconductor wafer over adefined active device area; next implanting a first dose of ions intothe active device areas; next forming a photosensitive material spaceralongside the gate, wherein forming the photosensitive material spacerincludes coating the gate with a layer of photosensitive materialsuitable for use as the spacer, flood exposing the photosensitivematerial for a controlled length of time, and developing thephotosensitive material, wherein flood exposing the photosensitivematerial for a controlled length of time, and developing thephotosensitive material avoids the use of a photo mask; next implantinga second dose of ions into the active device areas; and removing thephotosensitive material spacer.
 7. The method of claim 6, whereinremoving the spacers includes using an organic stripper selected fromthe group consisting of phenol-based strippers, low-phenal strippers,and phenol-free organic strippers.
 8. The method of claim 6, whereinremoving the spacers includes using a dry etching process.
 9. The methodof claim 6, wherein forming a gate on the semiconductor wafer includesforming the gate with a gate height, and wherein coating the gate with alayer of photosensitive material includes depositing the photosensitivematerial onto the wafer to a photosensitive material height, such thatthe photosensitive material height is greater than the gate height. 10.The method of claim 6, wherein the times of exposure and development aresufficient to remove photosensitive material from horizontal surfaces ofthe gate, but not long enough to remove the photosensitive material fromvertical surfaces of the gate.
 11. The method of claim 6, wherein floodexposing the photosensitive material for a controlled length of timeincludes irradiating the structure with ultraviolet light, wherein thewavelength of the ultraviolet light is between approximately 157 to 436nanometers.
 12. A method for forming a LDD structure on a semiconductorwafer, utilizing disposable photosensitive material spacers, comprising:forming a gate, having edges and a gate height on the semiconductorwafer; implanting a first dose of ions into the active device areas;forming a photosensitive material spacer alongside the gate, whereinforming the photosensitive material spacer includes coating the gate andsemiconductor wafer with a uniform layer of photosensitive materialsuitable for use as the spacer, exposing and developing thephotosensitive material both in the absence of photo mask; implanting asecond dose of ions into the active device areas; and removing thephotosensitive material spacer.
 13. The method of claim 12, whereinimplanting a first dose of ions into the active device areas isperformed subsequent to forming a photosensitive material spaceralongside the gate.
 14. The method of claim 12, wherein the LDDstructure is formed on a NMOS device, and implanting the first dose ofions includes implanting phosphorus at a dose of between approximately1×10¹³ phosphorus atoms/cm² and 2×10¹³ phosphorus atoms/cm², andimplanting the second dose of ions comprises arsenic at a dose ofapproximately 1×10¹⁵ arsenic atoms/cm².
 15. The method of claim 12,wherein the LDD structure is formed on a PMOS device, and implanting thefirst dose of ions comprises boron at a dose of between approximately1×10¹³ boron atoms/cm² and 2×10¹³ boron atoms/cm², and implanting thesecond dose of ions comprises boron at a dose of approximately 1×10¹⁵boron atoms/cm².
 16. The method of claim 12, wherein coating thesemiconductor wafer with a uniform layer of photosensitive materialincludes depositing the photosensitive material to a height which is atleast 0.5 microns greater than the gate height.
 17. The method of claim12, wherein flood exposing the photosensitive material for a controlledlength of time includes irradiating the structure with ultravioletlight, wherein the wavelength of the ultraviolet light is betweenapproximately 157 to 436 nanometers.